PhD Oral Exam - Shubham Bhagat, Physics

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The immense success of conjugated organic molecules (COMs) in the multi billion dollar OLED industry has equally raised the interest in conjugated polymers (CPs) for organic electronics. This is largely due to their flexibility and scalable, solution-based processing using cost-efficient methods. In most applications, the utility of such organic semiconductors (OSCs) in electronics relies heavily on tuning their electronic properties via doping. Unlike COMs, which can easily form highly ordered films via vacuum deposition, CPs are thermally fragile, thus, typically requiring solution processing. This often results in films with less-defined microstructures influenced by various experimental conditions. Further, CPs are susceptible to structural and chemical defects that reduce their conjugation length. This thesis employs the unconventional method of high-vacuum electrospray deposition (HV-ESD) to create poly(3-hexylthiophene) (P3HT) films under minimal solvent influence elsewise only applicable for COMs. Conventional solution-processed P3HT is typically semi-crystalline, comprising crystalline and amorphous regions as shown by X-ray diffraction. In contrast, it is found in this work that HV-ESD yields highly amorphous films. This technique allows therefore studying CP films without the complexity of mixed crystalline and amorphous regions, which is important in analyzing properties like the doping of P3HT, where these regions respond differently to doping and information gathered thereon is generally convoluted. Further, this thesis focuses on COMs exploring the optical and structural properties of blends created by the rod-like molecules α-sexithiophene (6T) and para-sexiphenyl (6P), which are prototypical OSCs. Vacuum co-deposited films yield a mixed crystal structure, confirmed by grazing-incidence X-ray diffraction, and unique optical properties. The blends show green emission alongside the characteristic red and blue emissions of pure 6T and 6P explained by J- and H-type aggregation. Additionally, this work includes studies addressing the molecular doping of oligomers to examine transitions between distinct doping phenomenologies based on the conjugation length, and the yet to be fully understood doping of P3HT by Lewis acids as alternative dopants. These studies are nearing publication. Overall, by commissioning HV-ESD as a tool to deliberately establish amorphous OSC films, and by assessing OSC blends and molecularly doped OSCs in thin films, this thesis contributes a broad view on the interplay between structural and optoelectronic properties of this important material class.